c#对象泛型使用没有铸造？

public class GenericClass<T>
{
    private T _data;

    public GenericClass(T data)
    {
        _data = data;
    }

    public T GetData()
    {
        return _data;
    }
}

public static T GetDefaultValue<T>()
{
    return default(T);
}

var intInstance = new GenericClass<int>(5);
var stringInstance = new GenericClass<string>("Hello, World!");

int defaultInt = GetDefaultValue<int>(); // 0
string defaultString = GetDefaultValue<string>(); // null

public class GenericClass<T> where T : class
{
    // ...
}

public static T GetDefaultValue<T>() where T : struct
{
    return default(T);
}